<!doctype html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport"
          content="width=device-width, user-scalable=no, initial-scale=1.0, maximum-scale=1.0, minimum-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
</head>
<body>
<button id="btn1">按钮1</button>
<button id="btn2">按钮2</button>
<button onclick="demo()">按钮3</button>
</body>
<script type="text/javascript">
    const btn1 = document.getElementById('btn1')
    btn1.addEventListener('click', () => {
        alert('按钮1')
    })

    const btn2 = document.getElementById('btn2')
    btn2.onclick = () => {
        alert('按钮2')
    }

    function demo() {
        alert('按钮3')
    }
</script>
</html>